The present invention relates to a semiconductor memory device and a method of fabricating the same.
A ferroelectric film is applied in a wide range of devices since it has many properties such as spontaneous polarization, high permittivity, electro-optical effect, piezoelectric effect, pyroelectric effect and so forth. For example, the ferroelectric film is used in various fields such as an infrared linear array sensor by utilizing its pyroelectricity, an ultrasonic sensor by utilizing its piezoelectricity, a waveguide optical modulator by utilizing its electro-optical effect and a dynamic random access memory (hereinafter, referred to as DRAM) by utilizing its ferroelectricity.
Above all, with the advancement of thin film formation techniques in recent years, a ferroelectric nonvolatile memory (FRAM), which has a high density and operates at a high speed, is actively being developed by using ferroelectric techniques and semiconductor memory techniques in combination. Nonvolatile memories using a ferroelectric film are actively being researched and developed for practical use as memories that can replace not only conventional nonvolatile memories, but also static RAMs (SRAMs) and DRAMs, due to their characteristics such as high-speed writing/reading, low-voltage operation and writing/reading resistance.
In order to develop such devices, a material is required which has properties such as high remanent polarization, low coercive field, low leakage current and high resistance to repeated polarization reversals. Furthermore, it is desirable to realize the aforementioned properties in a thin film having a thickness of 200 nm or less to match a lower operating voltage and a semiconductor fine processing process. As a ferroelectric material used for these devices, a bismuth lamella structure compound thin film such as lead zirconate titanate ((PbxLa1−x) (ZryTi1−y)O3 (0≦x, y≦1) (hereinafter, referred to as PZT)) and SrBi2 (TaxNb1−x)2O9 (0≦x≦1) (hereinafter, referred to as SBT) is suitable for application to ferroelectric and high dielectric integrated circuits.
Meanwhile, in order to increase capacitance of a capacitor in response to higher integration of DRAMs, high dielectric materials, which are materials having higher permittivity than that of a conventionally used silicon oxide film, such as tantalum oxide (hereinafter, referred to as Ta2O5) film, strontium titanate (hereinafter, referred to as SrTiO3), barium strontium titanate and so forth are to be used for future highly integrated DRAMs of 256 megabits to 1 gigabit or higher and are being actively researched and developed.
A conventional semiconductor memory device including a capacitor made of a ferroelectric film is shown in FIG. 4. This semiconductor memory device is fabricated as follows.
First, a gate oxide film 103 of a selection transistor for reading/writing a memory is formed on a conductive silicon substrate 101 having source/drain regions 104, 104 and a device isolation region 102, and a polysilicon word line 105 is formed on the gate oxide film 103.
Then, a material of an interlayer insulating film 106 is deposited on the polysilicon word line 105 and the conductive silicon substrate 101, and then Ti or an oxide of Ti, which is a material of an adhesion layer 107 for a Pt lower electrode 108, is deposited.
Then, the Pt lower electrode 108, a ferroelectric film 109 and a Pt upper electrode 110 are formed by dry etching to complete a dielectric capacitor consisting of the lower electrode 108, ferroelectric film 109 and upper electrode 110.
Then, a first diffusion barrier film 111 made of an oxide of Ti, Al, Zr or the like is formed so as to cover the whole ferroelectric capacitor. This first diffusion barrier film 111 prevents a reaction between the ferroelectric film 109 and a second interlayer insulating film 112 and diffusion of hydrogen generated upon the formation of the second interlayer insulating film 112 into the ferroelectric capacitor.
Subsequently, an interlayer insulating film layer 112 such as a silicon oxide film or the like is formed on the first diffusion barrier film 111.
Then, a contact hole 115 is formed to connect the Pt upper electrode 110 of the ferroelectric capacitor and the source/drain regions 104 of the selection transistor with a first metal wiring 113. Then, the first metal wiring 113 made of Al or the like is provided to interconnect the Pt upper electrode 110 and the source/drain regions 104.
Finally, a surface protection film layer 114 such as a silicon nitride film is formed and sintered at around 400° C. in an atmosphere containing 2-5% hydrogen as a final thermal treatment.
Ferroelectric films and high dielectric films are easily reduced when they are brought into contact with hydrogen, and metals such as Pt, Ir and the like, which are materials of an electrode in contact with a ferroelectric film or a high dielectric film, have a catalytic effect of promoting a strong reduction reaction. Therefore, when hydrogen is adsorbed to the electrode, hydrogen is activated and diffused into the ferroelectric film, and the ferroelectric film is easily reduced.
Generally, a process for fabricating a semiconductor memory device having a dielectric film in a capacitor includes a lot of process steps where hydrogen is generated. An interlayer insulating film formed on the high dielectric capacitor or the ferroelectric capacitor is usually formed by a chemical vapor deposition method (hereinafter, referred to as CVD method) using silane (hereinafter, referred to as SiH4) as a main source. In this case, when the interlayer insulating film is formed, the source is decomposed and hydrogen is generated. When this hydrogen is diffused into the ferroelectric capacitor or the high dielectric capacitor, the ferroelectric film or the high dielectric film is reduced, thereby increasing a leakage current or lowering the remanent polarization value. As a method of preventing deterioration due to the hydrogen diffusion, there is a method wherein the whole ferroelectric capacitor is covered with a first diffusion barrier film 111 made of an oxide of Ti, Al, Zr or the like as in the case of the semiconductor memory device shown in FIG. 4. In addition to this method, it is effective to cover the ferroelectric capacitor or the high dielectric capacitor with a diffusion barrier film made of an oxide of Ti, Al, Ta or the like as disclosed in Japanese Patent Laid-Open Publication Nos. 8-335673 and 10-294433.
Meanwhile, since a layout of memory cells and peripheral circuits is complicated in a semiconductor memory device having a degree of integration of one megabit or higher, two or more layers of metal wirings are required.
FIG. 5 is a cross sectional view showing a semiconductor memory device using two-layer metal wirings. In FIG. 5, the same component members as those in FIG. 4 are designated by the same reference numerals and their explanation is omitted.
First and second metal wirings 113, 213 shown in FIG. 5 are usual aluminum wirings. After the first metal wiring 113 in the first layer is formed, a third interlayer insulating film 212, which is a silicon oxide film, is provided by the CVD method. Then, after a material of the second metal wiring 213 in the second layer is deposited on the third interlayer insulating film 212, the second metal wiring 213 in the second layer is formed by a reactive ion etching method by using a photoresist as a mask. When an aluminum wiring in the third layer is formed, an interlayer insulating film is similarly provided on the second metal wiring 213 in the second layer, and the aluminum wiring in the third layer is formed on this interlayer insulating film.
Since the second and third interlayer insulating films 112, 212 are formed by the CVD method using SiH4 as a main source, the source is decomposed during the formation of the layers, and hydrogen is generated. Furthermore, since the second and third interlayer insulating films 112, 212 are formed at around 400° C., hydrogen remains in the films, and hydrogen is released from the second and third interlayer insulating films 112, 212 during a thermal treatment process after the formation of the second and third interlayer insulating films 112, 212 and further during a thermal process in the formation of the upper, third interlayer insulating film 212. Thus, since a large amount of hydrogen is generated during the process of forming aluminum wirings in a plurality of layers, that is, the process of forming the first and second metal wirings 113, 213, it is difficult to obtain a sufficient barrier property only by the first diffusion barrier film 111 formed so as to cover the ferroelectric capacitor, and hence a problem arises that a capacitor characteristic is deteriorated.
As a method of preventing this deterioration of the capacitor characteristic, a method may be conceived wherein a diffusion barrier film is formed so as to cover the first and second metal wirings 113, 213, thus preventing diffusion of hydrogen. However, since the covering ability of the diffusion barrier film for a stepped surface is poor as long as the diffusion barrier film is formed by a sputtering method, the thickness of the diffusion barrier film covering side surfaces of the first and second metal wirings 113, 213 becomes about 50-70% of the thickness of the diffusion barrier film on top of the first and second metal wirings 113, 213.
Furthermore, a method is also conceivable wherein the thickness of the diffusion barrier film covering the side surfaces of the first and second metal wirings 113, 213 is increased to prevent deterioration of the capacitor characteristic. However, since the diffusion barrier film is a dense film, its film stress is large. Thus, if the thickness of the diffusion barrier film is increased to secure its barrier property for the side surfaces of the first and second metal wirings 113, 213, reliability of the first and second metal wirings 113, 213 will disadvantageously be deteriorated.
Furthermore, the diffusion barrier film covering the first and second metal wirings 113, 213 is effective when it is formed from an oxide of Al, nitride of Al, oxynitride of Al, oxide of Ta, oxynitride of Ta, oxide of Ti or oxide of Zr. The film is formed by a reactive sputtering method using a mixed gas of Ar and oxygen. However, since oxygen atoms become negative ions during formation of the diffusion barrier film, charges are easily accumulated on the substrate or wafer. If such charges are accumulated in the first metal wiring 112 connected to the capacitor, a high voltage may be applied to the capacitor, resulting in a dielectric breakdown.